Multiple-input-multiple-output (MIMO) communication techniques are widely deployed in ground or near-ground wireless cellular and local area networks such as 4G LTE and Wi-fi networks. They provide substantial capacity increases compared to conventional single-input-single-output (SISO) systems. In cellular or Wi-fi environments, wireless channels between transmit antennas and receive antennas exhibit random fading due to rich scattering, which is often characterized as Rayleigh fading. It has been shown that the capacity of MIMO systems subject to Rayleigh fading increases linearly with the number of transmit antennas, provided that the number of receive antennas is not less than that of transmit antennas. The feasibility of applying the MIMO concept to airborne ad-hoc networks has been studied where aircraft or unmanned-aerial-vehicles (UAV) communicate with each other through multiple antennas carried within each aircraft. However, key challenges exist in airborne or free-space MIMO wireless communications, such as: (i) the absence of rich scattering and reflections; and (ii) the link between each transmit antenna and each receive antenna being an essentially line-of-sight Gaussian channel. Consequently, three-dimensional (3-D) spatial MIMO channels may be highly correlated as they induce a singular MIMO channel matrix, and thus may not offer the promising capacity increase compared to the conventional ground/near-ground MIMO wireless communications.
In view of the above, it is advantageous to provide a robust, spectrally efficient line-of-sight (LoS) communication system with geometrically-distributed antenna arrays.